1. Field of the Invention
The present invention relates to a fail-safe mechanism for an electric brake system for use on motor vehicles, and more particularly to a fail-safe mechanism for an electric brake system including braking force generating mechanisms for applying braking forces to road wheels, the braking force generating mechanism being mechanically separate from a brake pedal and having respective electric actuators that are independently energizable in response to depression of the brake pedal.
2. Description of the Relevant Art
Some motor vehicles incorporate electric brake systems comprising braking force generating mechanisms associated with the respective four road wheels. Such electric brake systems have a sensor for detecting the depression by the driver of the brake pedal. In response to a signal from the sensor, electric actuators of the braking force generating mechanisms are energized to apply boosted braking forces to the road wheels.
One known motor vehicle brake apparatus is disclosed in Japanese Laid-Open Patent Publication No. 60-206766 (corresponding to U.S. Pat. No. 4,658,939), for example. The disclosed motor vehicle brake apparatus includes an electric brake system comprising electric actuators for braking four road wheels of a motor vehicle, and a conventional fluid-pressure brake system for applying braking forces to the front road wheels. In the event of a failure of the electric brake system, the fluid-pressure brake system operates to brake the motor vehicle. When some of the electric actuators fail to operate, the entire electric brake system is put out of operation, and the fluid-pressure brake system is immediately available to brake the motor vehicle.
If the normal electric actuators were able to operate in the event of a failure of other electric actuators, then they would be effective to increase braking forces to be applied to the road wheels and hence to give better controllability to the motor vehicle when it is braked.